Large Eddy Simulation of PBL turbulence and clouds Chin-Hoh Moeng National Center for Atmospheric Research

OUTLINE 1.The LES technique 2.PBL turbulence and clouds 3.Role of LES in PBL research 4.Future direction

Numerical methods of studying turbulence Reynolds-average modeling (RANS) model just ensemble statistics Direct numerical simulation (DNS) resolve for all eddies Large eddy simulation (LES) intermediate approach

1. LES turbulent flow Energy-containing eddies Subfilter scale eddies (not so important) (important eddies)

Example: An 1-D flow field f Apply filter 

Reynolds average model (RANS) f Apply ensemble avg  non-turbulent

LES EQUATIONS SFS Apply filter G

The premise of LES Large eddies, most energy and fluxes, explicitly calculated Small eddies, little energy and fluxes, parameterized, SFS model LES solution is supposed to be insensitive to SFS model

Caution near walls: eddies small, unresolved very stable region: eddies intermittent cloud, radiation, chemistry… introduce more uncertainties

Major differences between geophysical and engineer flows inertial (vs. viscous) layer near walls (molecular term is always neglected) entrainment-into-inversion (vs. rigid top) buoyancy effect cloud processes

PBL ~ meters

2. WHAT IS THE PBL? turbulent layer –lowest ~km on the Earth surface directly affected by surface –heating, moisture, pollution, sfc drag diurnal cycle over land –convective and stable PBLs

PBL TURBULENCE dispersion transport ground temperature air-sea interaction global radiation budget via marine stratocumulus clouds

ANNUAL STRATUS CLOUD AMOUNT

~ 100% < 10% transition

marine stratocumulus off California coast persistent all NH summer!

from aircraft capped by a strong inversion

Stratocumulus-topped PBL ~ 50% < 10% ocean PBL

4% increase in area covered by PBL stratocumulus cloud 2-3 K cooling of global temperature (Randall et al 1984)

Stratocumulus-topped PBL cold ocean water PBL entrainment radiative cooling evaporation drizzle condensation Warm and dry aloft

two cloud-top processes radiation evaporation entrainment PBL cold ocean surface

cloud-top mixing process fluid a fluid b saturation point 1

ISSUES on marine stratocumulus PBL formation and dissipation processes? parameterization in climate model? cloud albedo? cloud amount or if global warming occurs?

Different PBL Regimes convective PBL stable PBL oceanic boundary layer shallow cumulus-topped stratocumulus-topped PBL over wavy surface …

3. LES of DIFFERENT PBL REGIMES Domain setup Large-scale forcing Flow characteristics

Clear convective PBL Convective updrafts ~ 2 km

The stable PBL

Oceanic boundary layer Add vortex force for Langmuir flows McWilliam et al 1997

Shallow cumulus clouds Add phase change---condensation/evaporation ~ 6 km ~3 km ~ 12 hr

How to include condensation/evaporation in LES? conserved variables

Stratocumulus-topped PBL Add latent heat and longwave radiation ~ 5 km ~1 km rad cooling cloud layer thin rad cooling layer >10K

F F height 0 Q_rad IR radiative fluxes O(100K/day)

How to include longwave radiation in LES?

LES vs. observation mean thermodynamic properties time evolution of cloud top, bottom w-variance and skewness

heat fluxes moisture flux buoyancy flux Z (m) cld top cld base

How do we study PBL turbulence and clouds with LES?

Study turbulence behavior and processes responsible for transport (creative thinking; flow vis.) Develop or calibrate ensemble- mean models (RAN models) (large database)

CLASSICAL EXAMPLES Deardorff (1972; JAS) - mixed layer scaling Lamb (1978; atmos. env) - plume dispersion property

Entrainment

Sullivan et al 1998 JAS

So far, idealized PBLs: Flat surface Periodic B.C. in horizontal Shallow cloud regimes

Challenge of LES for PBL Research Real-world PBLs: – complex terrain – complex land use – ocean waves – severe weather

4. FUTURE RESEARCH Extending LES applications to real-world PBL problems

Use a state-of-the-art weather model

Why Weather Research and Forecast (WRF) model? Available input data: –Terrain, land properties, meteorol conditions Higher-order numerical schemes Terrain-following coordinate Design for massive parallel computers – partition in vertical columns

500 km 20 km nest an LES inside the WRF model

Technical Issues Inflow boundary conditions SFS representation near irregular surfaces Proper scaling; how to represent ensemble statistics

? How to describe a turbulent inflow?

SUMMARY LES in advancing PBL research Marine stratocumulus in climate models Technical issues in extending LES to real PBLs